It has been difficult to isolate distinguishing features that would categorize specific dyslexia as a single condition. SLDs have many subtypes with a variety of breakdowns, as suggested by Carlson (1998). He suggested two major distinctions of people as follows:

Developmental SLDs: are caused by biological anomalies, usually genetic, in the brain at various levels (i.e. morphological or synaptic) from prenatal through childhood development.

Acquired SLDs: are caused by brain trauma that may occur prenatally or later, which leads to similar behavioral characteristics of developmental SLDs.

As mentioned previously, we will focus on dyslexia, which is a general term for reading disorders with many subgroups (i.e. surface, phonological, word-form or spelling, and direct). As a quick note, dysgraphia, writing disorders, has similar subgroups (i.e. surface, phonological, and direct). Basically, writing involves reading with an output expression using visual imagery and phonetics, as well as memory of letter sequence and muscle motor. The dyslexia subgroups can be understood as follows:

Phonological : person can read familiar words by using whole word method but has difficulty "sounding out" words that are new or letter-to-sound decoding problems.

Spelling : person can read individual letters that lead to reading words if given enough time but has problems recognizing the word as a whole and phonetically (i.e. men ---> h-e-n).

Direct : person can read aloud without comprehension that is similar to speech comprehension aphasias like anomia.

Since all dyslexics do not possess the same symptoms, Ingram (1964) suggested dividing dyslexia into three categories,

visuospatial difficulties

speech sound difficulties

correlating difficulties

Persons with visuo-spatial difficulties cannot recognize groups of letters. They may tend to guess words by shape and not by context. People with this form of dyslexia may also confuse reversible letters, transpose letters in syllables and syllables in words and words in phrases. They have trouble reproducing letters in writing and may confuse letter, syllable and word order. They may also read words backwards.

Those with speech sound difficulties have problems in understanding spoken language. Difficulty arises in breaking words into syllables and in forming sentences. Those who have problems correlating are unable to find the appropriate speech sounds for individual letter or sounds in writing (this is seen more commonly with monosyllabic words).
Researchers began to standardize the concept of dyslexia as case studies and investigations increased. The divisions below are listed according to the researcher of dyslexia. (Snowling Margaret, 1972)

Boder's- Reading-Spelling Pattern Dysfunction

Boder developed a diagnostic screening tool for developmental dyslexia from which she divided into three subtypes:

Dysphonetic- This is the largest of the three divisions. This is viewed as a disability in associating symbols with sounds. The misspellings typical of this disorder are phonetically inaccurate. The misreadings are substitutions based on small clues, and are also semantic.

Dyseidetic- In this group there are deficits in vision and memory of letters and word shapes. The person is unable to develop a sight vocabulary. However they have the ability to acquire adequate phonetic skills.

Alexic or mixed dyseidetic and dysphonetic- This subtypecombines the deficit of the first two groups. This person may have disability in both sight vocabulary and phonetic skills. People with this form of dyslexia are usually unable to read or spell.

Johnson & Myklebust's- Visual and Auditory

Johnson and Myklebust(1967) standardized reading tests as well as tests for reading diagnosis. They also identified subtypes of dyslexia.

Visual Dyslexia- Those with visual dyslexia usually cannot learn words as a whole component. The person has problems with visual discrimination, memory synthesis and sequencing of words. Reversal of words or letters when reading, writing and spelling is common.

Auditory Dyslexia- Auditory dyslexics cannot link the auditory equivalent of a word to the visual component.

Bateson (1968) identified three categories based on the Illinois test of Psycho linguistic Abilities. They are:

good visual memory but poor auditory memory

good auditory memory but poor visual memory

poor visual and auditory memory. In this case reading disability is severe and difficult to remedy deficits.

Smith's Three Patterns based on WISC and WAIS tests.
In 1970 Smith used the Weschler intelligence test and the WISC to draw some conclusions. Smith's inferences were similar to Border's. He detailed three pattern dyslexics:

those who have strong spatial ability also score lower in symbol manipulation and sequencing.

those who had problems in spatial organization and perceptual organization also have visual-motor coordination deficits.

those who have deficits characteristic of pattern i and ii.

Mattis, French and Rapin-Language Disorder group, Articulary and Dyscoordination Group and a Visualspatial Verceptual Disorder group.

Mattis French and Rapin conducted a study of 113 children to investigate the cause of dyslexia. They tested all of children on IQ, vision, hearing and academic exposure. They divided the children into three groups: (I) those with brain damage who could read, (II) those with brain damage who were dyslexic and (III) those without brain damage who were dyslexic. Interestingly enough they found a similarity between those with developmental dyslexia and those with brain-damage dyslexia. On the basis of these results and a battery of neuropsychological examinations, they divided dyslexia into three syndromes.

Syndrome II Articulatory and Graphomotor Dyscoordination- Children with this syndrome have gross and fine motor coordination disorders. They have poor speech and graphomotor coordination.

Syndrome III: Visuospatial Perceptual Disorder- These children score 10 points more on verbal IQ than performance IQ. Their visuospatial perception is very poor, as is their ability to store and retrieve visual stimuli. (Macdonald Critchely, 1990)